1. Field of the Invention
This invention relates to a method and an apparatus for achieving chemical-mechanical jet etching of features in a workpiece such as a semiconductor substrate or a built-up stack of films. In particular, the invention permits high etch rates for thinning the workpiece or for production of relatively large dimension etched features in such a workpiece, such as trenches and through-wafer vias for electrical contacts, or reservoirs for ink-jet heads, where critical dimensions are relatively large, on the order of 1-100 microns, or for thinning in the range of 100-1000 microns.
2. Brief Description of the Background Art
Various wet-chemical etching, dry-etching (i e., various forms of plasma etching) and electrochemical methods exist which permit creation of features on a semiconductor substrate or on a stack of layers of semiconductor, conductor, and dielectric materials, or similar flat structure, generally using various sorts of patterned masks to control where underlying material is etched away and where it is left in place. Such methods provide the required degree of precision for etching semiconductor features with critical dimensions in the submicron range, but typically achieve etch rates in the range of about 1 micron or less of thickness per minute. Where the required features are relatively large-scale, in the range of from about 10 to about 100 microns, as in making a through-wafer via or large trench on a silicon wafer, or the amount of material to be removed is large, as in wafer thinning applications, and the required precision is less critical, the etch rates achievable by such methods are generally too low to be practical in commercial production. What is needed is a method permitting the removal of large amounts of silicon wafer or other material, with moderate precision, at etch rates sufficiently rapid to be practical in industrial semiconductor processing.
We have invented a chemical-mechanical jet etching method which is capable of quickly and accurately removing large amounts of material from a workpiece in wafer thinning applications, and is capable of rapidly producing large-scale features, with critical dimensions on the order of 1-100 microns, and aspect ratios (defined as the ratio of the depth of an etched feature to the maximum diameter of a via, or the maximum width of a trench, at the entrance) in the range of 1:1 to 10:1. The method of the invention can be carried out on a generally flat semiconductor workpiece, including but not limited to a silicon wafer or gallium arsenide or other semiconductor substrate, silicon-on-insulator (xe2x80x9cSOIxe2x80x9d), SiO2, glass, quartz, pyrex, ceramic, or glass bonded to a substrate, conductor, or insulator. In particular, the method is expected to function very advantageously when applied to silicon wafer, gallium arsenide, and SOI substrates, In both thinning and feature creation applications, etch rates of at least about 10-100 microns of substrate thickness per minute can be achieved.
The method uses an ejector, typically a nozzle or array of nozzles, to deliver a jet, a high-pressure stream of a fluid etchant medium, referred to in this application as the xe2x80x9cmachining etchant,xe2x80x9d to the surface of a workpiece. Material is thereby machined from the surface, at a rate as high as about 100 microns of depth per minute, in areas of the substrate upon which the jet impinges. The areas which are not to be etched may be shielded from the jet by a patterned mask, or the jet may simply be directed at those specific areas from which material is to be removed, depending on the size of the desired feature or etching area. As described above, the desired features may be on a workpiece such as a silicon wafer, SiO2, gallium arsenide substrate, or silicon-on-insulator substrate. For example, and not by way of limitation, machined features created on a semiconductor substrate wafer may include a through-wafer via for making an electrical connection from one surface of the wafer to the other, or a large trench of the kind used in formation of conductor lines which run across a surface of the wafer, or other three-dimensional shape to be created on the workpiece. The method may also be used to remove material from the surface of a workpiece which is to be thinned to a smaller thickness.
The apparatus used to carry out the method may provide for the use of a single ejector, generally a nozzle or an array of nozzles. Each nozzle may be a single nozzle, or a dual nozzle, designed to deliver a compound jet of machining etchant to the workpiece The nozzle or nozzles may be stationary or movable. The workpiece may likewise be stationary or movable. The workpiece may be rastered, rotated, or translated at an angle with respect to a stationary nozzle or nozzles, or the nozzles moved with respect to a stationary workpiece, or both may be moved.
The machining etchant used to carry out the invention may be a chemically inert liquid, which merely acts to bombard the surface, or a liquid solution of a typical chemical etchant, which bombards but also chemically reacts with the surface. The machining etchant may be a slurry, including a suspension or dispersion, of abrasive or other solid particles in a liquid or a gas medium. The slurry may include a chemical etchant, or may rely solely on the presence of solid particles to provide physical bombardment. The chemical etchant, if used, may be a liquid, or it may be a solution of an etchant compound dissolved in the liquid component of the slurry, or it may be some or all of the solid particle component of the slurry. In the case of the suspension or slurry, the particulates present in the machining etchant can be of any appropriate chemical composition, particle size, particle size distribution, or shape. The machining etchant may also be a dry powder, used for physical bombardment and/or chemical reactivity, and carried by a high-pressure gas stream.
Jet etching can be done by direct-writing to the surface to be etched, or using a mask, patterned so as to permit impingement of the etchant only in those areas of the workpiece where material is to be removed. The mask, if any, covering that portion of the workpiece surface where no etching is desired may be a photoresist mask, an inorganic hard mask, or other protective mask. Following jet etching, the mask may be removed, if desired, using an appropriate solvent and/or dry etch process, as applicable, to remove residual mask material.
In one exemplary embodiment of the invention, the workpiece is a silicon or other semiconductor substrate wafer. The machining etchant is a slurry of abrasive or physical bombardment particulate material, carried in a liquid, which may also be a chemical etchant or solution of a chemical etchant. The particulates may be selected from among the following materials: fumed silica, cerium oxide, alumina, diamond, green or black silicon carbide, boron cagernet, or other similar fine-grained hard materials. Specific materials are expected to be particularly suitable for use with specific workpiece compositions, as is known to persons skilled in the semiconductor art. The liquid may be selected from among the following materials: Water can be used where minimal or no chemical component, but only mechanical etching, is desired. The chemical etchants that may be used as the slurry liquid are selected from among solutions of the following materials: KOH, NaOH, HF, HNA (an aqueous solution of about 7 wt % HF, about 30 wt. % HNO3, and about 10 wt. % CH3COOH), TMAH (tetramethyl ammonium hydroxide), EDP (ethylene diamine pyrochatechol), and amine gallates. Again, specific etchants are expected to be particularly suitable for carrying out the invention on specific workpiece compositions, as known to persons skilled in the semiconductor art. In this embodiment, the nozzles are stationary, and are arrayed across a diameter or a radius of the (usually circular) workpiece wafer. A mask is applied over the surface of the workpiece wafer, and the wafer is rotated beneath (or above) the nozzle array such that the machining etchant impinges upon non-masked areas on the wafer surface facing the nozzles during each rotation of the wafer. Various methods may be used to detect when the silicon wafer material in the non-masked areas has been removed to the desired depth of a trench or other feature on the wafer surface, or when a desired hole has been etched all the way through the wafer in order, for example, to create a via extending from one surface of the wafer through to the other.